Conventional wireless systems employ radio-frequency (RF) transmitters to produce an output signal that can be applied to an antenna for communication between stations separated by some distance. In mobile wireless networks, one station may be a mobile station (MS), whereas another station may be a base station (BS). As the MS moves throughout the coverage area of the wireless network, the path loss between the MS and the BS changes due to a number of factors including the change in distance between the stations as well as the presence of objects in the environment that serve to obstruct or attenuate the signals traveling from one station to the other.
To ensure proper network operation, the BS instructs the MS to increase or decrease its transmit power as required to overcome the path loss between the MS and BS so that the BS will continue to receive the MS signals as channel conditions change. The MS must maintain a certain signal strength at the BS so as not to inhibit detection of its transmited signals. Depending upon the details of the physical environment between the MS and BS, at some critical distance from the BS the MS will no longer be able to increase its output power to maintain the required signal strength. At that point, communication between the MS and BS can no longer be maintained and the link will be dropped unless the BS is able to hand-off communication with the MS to a neighboring BS. Therefore, the maximum output power capability of the MS is a critical parameter that ultimately determines the expected distance over which the MS and BS can communicate and thereby the number and spacing of BS sites that is required to provide reliable coverage in a mobile network. However, the greater the number of BS sites, the greater the cost to implement the mobile network. Accordingly, there is a need to maximize the output power capability of the MS to ensure reliable coverage with a minimum of required BS sites.
Therefore, a critical component in a MS is the power amplifier that is used to transmit the signal to the BS. A power amplifier typically has a maximum output power rating. One method to attain reliable communication with a BS is to ensure that the power amplifier is equipped with sufficient power to overcome the fading and otherwise poor environment that is sometimes present in a wireless medium.
However, it is not always feasible to equip an MS with a high-power amplifier for several reasons: (i) there is a limit on the total power that may be consumed by the device; (ii) a high-power amplifier may get excessively warm and be inefficient; (iii) a high-power amplifier may be expensive; (iv) a high-power amplifier may be too large to fit within the size constraints of a small mobile terminal.
In an effort to increase the output power of the MS, rather than increase the power of an existing amplifier, it is sometimes economical and efficient to combine one or more lower-power amplifiers. Combining the outputs of multiple amplifiers may afford additional advantages, including the ability to transmit either a single spatial stream or a plurality of spatial streams.
It is desirable to have a method of optimizing transmission power of multiple power amplifiers of subscriber station transceivers that have multiple antennas.